Difference Between Carnot and Rankine cycle

Posted on December 12, 2011 by admin Last updated on: December 12, 2011

Carnot vs Rankine cycle

Carnot cycle and Rankine cycle are two cycles discussed in thermodynamics. These are discussed under heat engines. Heat engines are devices or mechanisms that are used to convert heat into work. Carnot cycle is a theoretical cycle, which gives the maximum efficiency that can be obtained by an engine. Rankine cycle is a practical cycle, which can be used to calculate real life engines. It is vital to have a proper understanding in these two cycles in order to excel in thermodynamics and any field relating to it. In this article, we are going to discuss what Carnot cycle and Rankine cycle are, their definitions, their applications, the similarities between Carnot cycle and Rankine cycle, and finally the difference between Carnot cycle and Rankine cycle.

What is Carnot Cycle?

Carnot cycle is a theoretical cycle, which describes a heat engine. Before explaining the Carnot cycle, few terms have to be defined. Heat source is defined as a constant temperature device, which will provide infinite heat. The heat sink is a constant temperature device, which will absorb infinite amount of heat without changing the temperature. The engine is the device or the process, which converts heat from the heat source to work. The Carnot cycle consists of four steps.

1. Reversible isothermal expansion of the gas – The engine is thermally connected with the source. In this step, the expanding gas absorbs heat from the source and does work on the surroundings. The temperature of the gas remains constant.

2. Reversible adiabatic expansion of the gas – The system is adiabatic meaning no heat transfer is possible. The engine is taken out of the source and insulated. In this step, the gas does not absorb any heat from the source. The piston continues to do work on the surrounding.

3. Reversible isothermal compression – The engine is placed on the sink and thermally contacted. The gas is compressed so that the surrounding is doing work on the system.

4. Reversible adiabatic compression – The engine is taken out of the sink and insulated. The surrounding continues to do work on the system.

In the Carnot cycle, the total work done is given by difference between the work done on the surroundings (step 1 and 2) and the work done by the surroundings (step 3 and 4). Carnot cycle is the most efficient heat engine in theory. The efficiency of the Carnot cycle depends only on the temperatures of the source and sink.

What is Rankine Cycle?

Rankine cycle is also a cycle, which converts heat into work. The Rankine cycle is a practically used cycle for systems consisting of a vapor turbine. There are four main processes in the Rankine cycle

1. The working of fluid into high pressure from a low pressure

2. The heating of the high pressure fluid into a vapor

3. The vapor expands through a turbine turning the turbine, thereby generating power

4. The vapor is cooled back inside the condenser.

What is the difference between Carnot Cycle and Rankine Cycle?

• Carnot cycle is a theoretical cycle whereas the Rankine cycle is a practical one.

• Carnot cycle ensures the maximum efficiency under ideal conditions, but the Rankine cycle ensures the operation in real conditions.

• The efficiency obtained by the Rankine cycle is always lower than that of the Carnot cycle.